The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus for transferring toner images formed on a plurality of image supporting members to a transfer member to form a color image.
In a conventional image forming apparatus of an electro-photography type such as a printer, a facsimile, a copier, a multi function product (MFP), and the likes, there is provided a printing mechanism. In the printing mechanism, after a charging roller charges an image supporting member (a photosensitive drum), an exposure unit exposes the photosensitive drum to form a static latent image thereon. After the static latent image is formed on the photosensitive drum, a developing roller attaches toner or developer to the static latent image with an electrostatic force to form or develop a toner image. Afterward, a transfer roller transfers or prints the toner image to a printing medium.
In the conventional image forming apparatus, a plurality of the printing mechanisms is provided for forming toner images in colors. A transportation belt or a transfer belt is provided for transporting the printing medium, and the toner images in colors formed with the printing mechanisms are transferred and overlapped on the printing medium, thereby forming a color image.
In the conventional image forming apparatus described above, when a color shift (i.e., a shift in printing positions of the toner images in colors) occurs, printing quality is deteriorated. In order to prevent the color shift, in the conventional image forming apparatus, when the toner images in each color are transferred and overlapped on the transportation belt, it is configured to detect an amount of the shift in the printing positions of the toner images in each color on the transportation belt. Accordingly, an operation of the printing mechanisms is controlled according to the amount of the shift in the printing positions thus detected (refer to Patent Reference). Patent Reference: Japanese Patent Publication No. 2001-134041
In the conventional image forming apparatus described above, a gear is generally provided for engaging with another gear provided on the photosensitive drum to drive the photosensitive drum as the image supporting member. The gear is normally formed of a multiple stage (for example, a two-stage gear) for decelerating and transmitting rotational drive from a drive source to the photosensitive drum. The gear for driving the photosensitive drum tends to exhibit a specific characteristic such as a deviation due to a dimensional accuracy of the gears engaging with other or a waggle tolerance of the gear from an axial center thereof.
When the gear for driving the photosensitive drum has the specific characteristic or a gear specific characteristic, the gear specific characteristic tends to influence on a rotation of the photosensitive drum, thereby causing a variation in the rotation of the photosensitive drum. The variation in the rotation of the photosensitive drum may be transmitted to the transportation belt. When the variation is transmitted to the transportation belt, a moving speed of the transportation belt may be fluctuated, thereby causing the color shift (i.e., the shift in the printing positions of the toner images in each color). Accordingly, the conventional image forming apparatus has the configuration in which the color shift tends to easily occur due to the influence of the gear specific characteristic.
In the conventional image forming apparatus described above, when a pitch distance is set at an optimal level, it is possible to prevent the color shift through a simple control of the printing mechanisms. More specifically, when the pitch distance is set at the optimal level, a transportation distance of the printing medium relative to a cycle of a speed fluctuation of the photosensitive drum becomes a fraction of an integer of the pitch distance.
In the conventional image forming apparatus described above, however, it is difficult to set the pitch distance at the optimal level due to a restriction of an apparatus size and a cost. In this case, since the pitch distance is not set at the optimal level, it is difficult to prevent the color shift through a simple control of the printing mechanisms. In other words, in the conventional image forming apparatus described above, when it is difficult to set the pitch distance at the optimal level, it is difficult to prevent the color shift through a simple control of the printing mechanisms.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus and preventing the color shift through a simple control of printing mechanisms even when it is difficult to set a pitch distance at an optimal level.
Further objects and advantages of the invention will be apparent from the following description of the invention.